It's an older report...but it might help since devices are getting bigger and the need to keep the cards alive longer is getting more and more important

My take on the document is that 4MB blocks or when writing the image
Write to SD card using Linux:
- Find the device (eg: /dev/sdb1) you need to write to by using blkid or mount.
- Double check that you have the correct dev path for your SD card (eg: /dev/sdb1).
- Unmount the SD card and all its partitions by using umount /dev/sdb?.
- Write the image using dd if=/path/to/DietPi_vXX.img of=/dev/sdb.